In the sandy, shallow habitats of Mamala Bay in O‘ahu,
Avrainvillea amadelpha has formed thick communities that cover the
substrate, invading the reef environment and out competing other algae and
our native seagrass, Halophila hawaiana.

Distribution of
Avrainvillea amadelpha in Hawai‛i
from surveys 1999-2000.

Avrainvillea amadelpha in the field.

Description

Plant consists of one to four wedge shaped blades that are
thin, diaphanous, 1 - 4 cm wide, and 1 - 3 cm tall. Each blade is attached
by a stipe, 0.4 - 1.5 cm long, to compact basal holdfast. Blades are
asymmetrical, surfaces felt-like, and margins smooth to lacerated. The stipe
is flattened in cross section. Color is green to green-gray, and
clumps are often covered with silty sand, appearing muddy brown.

Plants may be found singly or in clumps of many blades. In
larger, more mature communities, other macroalgae will be found attached to
blades.

Avrainvillea amadelpha is abundant in habitats of
shallow, sandy substrate with low water motion. Forms dense clumps often
covered with silt and sand. Overgrows coral rubble. Found from 1 to at least
10 meters deep.

Distribution

Hawai‘i: O‘ahu: Kahe
Point, Koko Head, Waikiki area, Diamond Head.

Mechanism of Introduction: First identified in shallow water locales at Kahe
Point and Koko Head. Thought to have arrived sometime after 1981. Deep water
populations collected by Hodgson at 40 ft in Waikiki. Actual mechanism of
introduction not known.

Worldwide Distribution:

Mauritius,
Tuamotus, Fiji, Philippines.

Ecology/Impact

Avrainvillea amadelpha is apparently a fairly recent
introduction to Hawai‘i. In established communities, this green alga has
invaded and now covers a large part of the substrate, becoming a secondary
substrate for other reef algae. Growth proceeds from the basal region rather
than from the blades, so plants are usually in tight clusters of single
blade height.

First found in 1981 on the leeward shore of O‘ahu, it is now
found in large communities with Acanthophora spicifera along the
southern shore of O‘ahu. Sandy bays once known for large native seagrass
beds, (Halophila hawaiiana) are now overgrown with these invasive
species. This alga’s closely packed blades trap sediments and provide
habitat for filter feeders such as worms and molluscs. With enough time,
these trapped sediments added fine silt to the sandy bottom and create a mud
layer upon the sand, thus changing the nature of the substrate.